1. Field of the Invention
The present invention relates to a radar or sonar apparatus. It is particularly concerned with such an apparatus including digital processing of the radar or sonar signals received
2. Summary of the Prior Art
Radar scanners have been manufactured since the 1940's. The development of radar has diverged into two methods, non-coherent pulse radars and Doppler radars. Doppler radars have received considerable development for military and aviation use. Modern Doppler radars make extensive use of digital signal processing and digital control. However pulse radars have remained extensively analog, using logarithmic receivers and analog gain controls to generate baseband video that is sent to the display for processing. The display converts the baseband video to digital form using a 1-bit comparator. Any signal processing that takes place in such displays is limited by the thresholding that take place in the comparator. Analogue processing and control has hereto been cheaper and adequate for most purposes.
Radar receivers have historically been based upon down conversion from S-band or X-band to an intermediate frequency (IF) of the order of 60 MHz. The signal at the intermediate frequency (IF) is then subject to IF filtering and analog logarithmic demodulation to baseband “video”, followed by baseband filtering. The radar signal processor is often placed in radar display apparatus, rather than the scanner itself. This is because there are size, weight and power constraints on the radar scanner enclosure. The scanner is exposed to the worst of the environment, whereas the display enclosure is often placed in a more benign environment such as a heated cockpit. When multiple displays are used, the radar information is typically sent out over a network, the radar signal processors, in the each display apparatus not connected directly to the scanner, are idle and redundant.
Analog IF filters are used for conventional mono-pulse radars. The optimum bandwidth for the IF filter (BW)=l/T, where T is the pulse-width.
The pulse-width is optimised for range resolution and average power. Short pulses are used for short range, where the highest range resolution is needed and the return signals are strongest. Long pulses are used when the absolute range resolution can be reduced, but higher transmit power is needed due to the lower power of the return signal.
The returned signal power is:s=k/R^4Where k is a constant and R is range
The range resolution of the display limits the displayable range resolution to a maximum proportion of the range say 1/1024. Thus at longer ranges the absolute range resolution can be reduced. It should be noted that increasing the range resolution reduces clutter; the excess number of range bins can be combined in the signal processor prior to display. Pulses of this optimum transmitted pulse-width are selected when the user changes instrumented range.
When a large number of different pulse-widths (say 8) are available, providing matched analog filtering becomes onerous to set-up, and liable to drift. Hence it is normal to provide a compromise sub-set of filter bandwidths, and use each one to cover several pulse widths. This leads to sub-optimal filtering. Analog video filtering follows detection to remove the spurious products that arise from the use of a logarithmic detector. The analog video loss-pass filter works at baseband and is required to have wide bandwidth to cut-off frequency ratio, yet have linear phase, so video filtering is minimal.
The radar signal requires conversion from analog to digital form using an Analog to Digital Converter (ADC) or more often just a comparator. The signal is required to be in digital form for use with a raster graphic display such as a CRT or LCD. This is because it has to be (scan) converted from polar (Range, Azimuth) to Cartesian co-ordinates (x, y). Prior to conversion low-pass video filtering is required to avoid aliasing artefacts. The log processing of the amplitude from a conventional radar receiver is a non-linear process, the output of which comprises many harmonics that have to be removed to avoid aliasing in the ADC. However as discussed above, the video filtering is minimal. The poor removal of these spurious products causes aliasing, and destroys information that could have been used in later signal processing, for enhancing the quality of the displayed information. The logarithmic baseband video precludes any signal processing that makes use of linear signals for example, Fast Fourier Transforms (FFTs).
Logarithmic detector receivers have historically had advantages in radar receivers due to their large dynamic range and are inherently CFAR (Constant False Alarm Rate). To ensure that these qualities are not compromised in a linear receiver, the dynamic range of the analog and analog-to-digital conversion must be very wide. Low noise pre-amplifier components, a high speed, high resolution (large numbers of bits) analog-to-digital conversion, very fast digital filters and ideally a floating-point processing digital signal processor (DSP) must be used. These have been expensive and difficult to use. Thus linear detection and processing have net previously been considered, being both uneconomic and unreliable (due to high power dissipation).
Dual range scanners are available, which allow the display of separate radar plan position indicators (PPIs), which are the conventional radar displays. However, in such scanners the scanner makes more than one full rotation using each pulse type, with the receiver optimised for the reception of one pulse type at a time. The displays from each range have the disadvantage of clearly not being simultaneously updated, which creates an ambiguity. Time is allowed for the receiver and transmitter characteristics to be changed, resulting is a period when no pulses are transmitted or received on any range. There is a resulting loss of information.
FIG. 1 of the accompanying drawings shows a known radar apparatus. It comprises five principal components, namely a processor 10, a transmitter section 20, an antenna structure 30, a receiver structure 40 and a display structure 50. The processor 10 generates pulse initiation signals which are passed via a digital bus 11 to the transmitter section 20. The processor also generates signals for controlling the receiver section 40, which are passed from the processor 10 to the receiver section 40 via a second digital signal bus 12.
The pulse initiation signals from the processor 10 are received at a pulse duration unit 21 in the transmitter section 20, which determines the pulse width of the pulses to be generated. The pulses are initiated by an edge of the pulse initiation signal, and their duration is thus fixed. The resulting pulse information is passed to a modulator 22 which drives a magnetron transmitter 23. That magnetron transmitter 23 is normally a vacuum device that produces high power microwave pulses, which will form the radar signals. Those microwave pulses are passed from the magnetron 23 via a band pass filter 24 which controls spurious emissions from the magnetron 23 to a circulator 25. That circulator acts as a switching unit and, at appropriate times, passes the microwave pulse to an antenna 31, from which they are transmitted. The antenna 31 is arranged to rotate, and has a rotary joint 32 and a motor 33 which drives the antenna at a predetermined rotation speed. The motor 33 is driven from a drive 34 which is powered from the modulator 22. The rotary joint 32 acts as a microwave connection between the rotating antenna and the circulator 25.
When return signals are received at the antenna 31, they are passed via the circulator 25 to a low noise converter 41 which converts the signals to an appropriate frequency. Generally, the magnetron will produce pulses in the X-band region sent on 9.4 GHz, in which case the low noise converter 41 will convert the received X-band signals to an IF frequency, such as 60 MHz. Note that the circulator 25 switches between the pulses for transmission from the magnetron 23 and the received signals received by the antenna 31 which are passed to the low noise converter 41. The signals from the low noise converter 41 are passed to a PIN diode attenuator 42 which is controlled by a Time Varying Gain (TVG) generator 43 which is controlled by the processor 10 on the basis of the signals passed via bus 12. That TVG generator 43 controls the gain of the receiver section 40 to compensate for range variation of the signal received by the antenna 31. The TVG generator 43 also controls a variable amplifier 44 which receives the output of the PIN attenuator 42 and controls the IF gain of the received signal. The output of the variable gain amplifier 44 is passed to a log detector 45 which generates an output which is the logarithm of the envelope with a received signal. That output is passed to a selectable band filter (video filter 46) which generates the output to the display section 50.
As illustrated in FIG. 1, the display section contains multiple display structures, each of which comprises a comparator 51, a spoke buffer 52, a signal processor 53 and a graphical display 54. However, in the arrangement shown in FIG. 1, all but one of those multiple display structures is mostly redundant. Thus, in FIG. 1, the output of the video filter 46 is received by a first display structure 55, comprising comparator 51, spoke buffer 52, signal processor 53 and display unit 54. The comparator 51 generates a digital output that changes when the input signal crosses a predefined voltage threshold. The spoke buffer 52 then receives the output of the comparator 51, and stores a digital representation of the signal received as a function of time. The digital signals stored in the spoke buffer 52 are then processed by the signal processor 53 to generate a signal to the display unit 54. However, those signals to the display unit 54 are also passed directly to the display unit 54 of a second display structure 56. In that display structure 56, the comparator 51, spoke buffer 52 and signal processor 53 are redundant. It would similarly be possible to provide more display structures operating a similar way.
In a sonar system, the structure is similar but the antenna is replaced by a sonar transducer which transmits and receives the sonar signals. That transducer does not rotate, unlike the antenna 31. Moreover, it would normally be desirable for the magnetron 23 to be replaced by a high power RF pulse generator. In addition, since the velocity of proportion of acoustic waves in water is substantially slower than radio waves propagating through air, and the maximum range of targets to be detected by a sonar system is normally less than the maximum range to be detected in a radar system, the pulse repetition intervals and the pulse widths used in a sonar system will be different from those used in a radar system.